Push-pull electron tube system



Oct. 24, 1944.

w. E. GILBERT 2,361,282

PUSH-PULL ELECTRON TUBE SYSTEM Filed NOV.'13, 1941 JM# A lIl HTTYJ.

Patented Oct. 24, 1944 PUSH-PULL ELECTRON TUBE SYSTEM Walter E. Gilbert,Philadelphia, Pa., asslgnor to Philco Radio and Television Corporation,Philadelphia, Pa., a corporation of Delaware Application November 13,1941, Serial No. 418,987

(i Claims.

'This invention relates to an improved phaseinverter circuit forcoupling ar single-tube driver stage to a push-pull amplifier withoutthe use of an input transformer or an additional phaseinverter tube. Y

In general, there are two methods of coupling a single amplifier tubewith push-pull tubes, first, by means of a'transformer having acentertapped secondary winding, and second, by means of an electronicphase-inverter circuit. The first class of circuit requires the use of atransformer which, if `its frequency characteristic is. to be good, isan expensive piece of apparatus. As is known in the art, the secondclass of circuit ordinarily uses a separate phase-inverter tube, theoutput of the single-tube driver stage being fed directly to the grid ofone of the push-pull tubes and to the grid of the other of the push-pulltubes through the phase-inverter tube. In this circuit there issubstantially no gain in the inverter tube circuit, the signals appliedto the grids of the push-pull tubes are substantially equal and oppositein phase, and the same phase relations are obtained as though atransformer were used. An additional tube is, therefore, introducedwithout resulting benefits in gain.

An improvement in inverter circuits which requires no input transformeror additional inverter tube, but in which the exciting voltage for thesecond push-pull tube is obtained from a screen grid of the firstpush-pull tube, is described fully in U. S. Patent No. 2,109,021 to E.L. Clark, issued February 22, 1938. The present invention is animprovement -on the circuit therein described, and is intended toproduce an even more efcient system of driving push-pull tubes.

One object of the invention, thereforais to couple a single-tubeamplifier stage with a pushpull amplifier stage without the use of atransformer or phase-inverter tube.

Another object of the invention is to balance the output of thepush-pull tubes by applying voltages to their grids which aresubstantially equal in magnitude.

Still another object of the invention is to prevent screen currentsaturation of one of the pushpull tubes at the positive end of itsswing.

A further object of the invention is to obtain a greater useful outputfrom the amplifier circuit without increasingthe distortion, and toobtain greater outputat the same values of distortion.

Other objects of the invention will be apparent from the followingdescription and the accompanying drawing, in which:

Fig. 1 is a diagrammatic illustration of an amplifier circuit embodyingthe invention; and

Fig. 2 illustrates a modified form of the invention.

Referring specifically to Fig. 1, there is shown a single driver tube Iwhich may be a voltage amplifier of any suitable type, such as a triode,as illustrated. or apentode. The anode of tube I is connected through asuitable resistor 2 to a' source of anode supply voltage. The condenser3 serves to couple the anode of the tube I to the control grid 4 of thefirst of the push-pull output tubes 5 and 6. The grid 4 may be connectedto a source of bias voltage through a suitable high resistance 1. Theanodes of the push-pull output tubes may be connected in the usualmanner to the primary 8 of the output transformer, and the secondary 9of this transformer may be connected to the load, e. g. to a loudspeaker I0. The tubes 5 and 6 are both shown to be of the pentode type,although as will be explained later, it is only necessary that the tube5 be other than a triode and this might be either a tetrode, pentode, orsome other of the known multi-grid type tubes. The screen grid of thepush-pull tube 5 is connected to the positive high voltage B supplythrough two resistors II and I2, the resistances` of which are,ingeneral, small compared to the plate resistance of the tube 5.

'Ihe signal voltage to be applied to the control grid of tube 6 isobtained at the junction of the resistors Il and I2, the control grid I3being connected to this junction through the coupling condenser I4. Biasvoltage may be applied to the grid I3 through a resistor I5 connected tothe same source of voltage used to bias the grid 4. The screen grid ofthe tube 6 may conveniently be connected directly to the source ofpositive B voltage.

In the operation of the amplifier circuit constructed in accordance withthe invention, the signal output of tube I is applied, by way of thecoupling condenser 3, to the control grid 4 of the pentode 5. Becausethere is a substantial control-grid-to-screen-grid transconductance intubes of this character, signal current will flow not only in the anodecircuit, but also in the screen circuit., And because of the presence ofthe serially connected load resistors II, I2 in the said screen circuita signal voltage will appear on the screen electrode and at the junctionof the resistors II and I2. This voltage will, of course, besubstantially degrees out of phase with the control grid voltage, as isfully described in the aforementioned Clark patent.

' auxiliary electrode is provided.

the outputs of tubes 5 and 6 will, therefore, be 10 substantially equaland 180 degrees out of phase, and proper push-pull operation will beobtained. In one particular embodiment of the invention, the driver tubewas a 7C6; the push-pull tubes were 7B5s; the resistors 'I and I5 wereeach 470,000 ohms; the resistors II and I2 were 10,000 and 6600 ohmsrespectively; and the condenser I4 was .006 mf.

While tubes 5 and 6 are illustrated as pentodes, the tube 6 may be atriode, omitting merely the suppressor grid and screen grid connections,while the tube 5 may be a tetrode, omitting the suppressor gridconnection. Obviously, any other forrn of tube may be employed in whicha suitable It is to be understood, of course, that if the tubes are of adifferent type, e. g. one a triode and the other a pentode,r thecharacteristics should be matched such that they will give satisfactorypush-pull operation.

Although resistors I I and I2 are shown as separate resistances, it willbe understood that they constitute an impedance having an intermediatetap point.

In the form of invention shown in Fig. 2 similar reference charactersdenote parts equivalent to those of Fig. l. sistor I6 has a valueapproximately equal to the sum of resistors II and I2 y,in Fig. 1, andthe reduced voltage is obtained by the voltage divider` consisting ofresistors I'I and resistor I5. It is apparent that the resistor I6. hasIthe same D. C. voltage drop thereacross as the sum of the voltagesacross resistors II and I2 of Fig. 1, and that the A. C. voltageappearing on the screen grid of tube 5 is properly reduced by thevoltage divider effect of resistors I1 and I5, and applied to tube 6. Itmay also be found advantageous to use an iron core choke instead of theresistor I6. The same swing may then be obtained while using a higher D.C. screen voltage, due to the lower ,voltage drop of the choke ascompared to the resistor I6, and even greater output may be obtained.The same applies to the resistors II and I2 of Fig. 1.

In operation, the circuit herein described has the advantage over thecircuit-shown in the aforementioned Clark patent in that there is notendency on the part of the screen circuit of the first push-pull tube 5to swingfbeyond the linear portion of its operating characteristic. Theaddition of the resistor I I (or the use of the relatively largeimpedance I6) lowers the voltage applied to the screen grid butsubstantially increases thc range and linearity of its characteristic.The resulting lower screen voltage, due to the use of the resistor II,will reduce very slightly the output of tube 5, but the improvedlinearity of the screen circuit substantially increases the availableoutput from the dependent tube 6. In one embodiment the net result ofthe use of this invention was an increase in overall output ofapproximately 10% at 10% distortion, and an increase in overall outputof 30% at the maximum usable output of the circuit.

It will be apparent that the invention is capable In this modificationthe reoi' further embodiments or modifications and is not limited to theparticular embodiments illustrated.

I claim:

1. An amplifier circuit, comprising a pair oi pentode electron tubesarranged for push-pull operation, a driver tube therefor. meansconnecting .the control grid oi' the first of said pentode tubes to theoutput circuit of said driver tube, an impedance in the screen gridcircuit of the ilrst pentode, and means connecting the control grid ofthe second of said pentode tubes to an intermediate point of saidimpedance, the portion of the impedance between said intermediate pointand the screen grid ol' the first pentode substantially reducing thevoltage on said screen grid and allowing the screen grid Ia greaterpositive voltage swing, thereby permitting the derivation of a greaterdriving voltage for the control grid of the second pentode withoutexceeding the limits of permissible distortion.

2. A phase inverter System for audio amplifiers and the like, comprisingan input stage having an unbalanced output circuit, an output stageincluding an independent space discharge device and a dependent spacedischarge device, said devices having their output circuits connected inbalanced relation, means for coupling the output circuit oi' said inputstage to the input circuit of said independent device, an auxiliaryelec-- trode disposed in the space current path of said independentdevice, a source of D. C. supply voltage, a connection between thecathode element of said independent device and the negative terminal ofsaid source, a pair of resistors connected in series between saidauxiliary electrode and the positive terminal o1' said source, and meansfor deriving the signal voltage developed across one of said resistorsand i'or applying said signal voltage to the input circuit of saiddependent device, the resistance o1' said one resistor being selected toprovide an input signal for said dependent tube which is approximatelyequal in magnitude to the signal applied to said independent tube, theresistance of the other resistor being suicient, when taken incombination with said one resistor, to insure substantially linearoperation of the auxiliary electrode circuit within the desiredoperating range.

3. A phase inverter system for a push-pull amplier, comprising a pair ofspace discharge devices, each having an anode, a cathode, and a controlelement, and at least the rst of said devices having an auxiliaryelectrode interposed between the control and anode elements, a loadcircuit connecting the anodes of said devices in push-pull relation,connections for applying an input signal to the control element of saidilrst device, an auxiliary circuit interconnecting the auxiliaryelectrode of Ysaid first device with the cathode of said rst device andincluding a source of direct current, said auxiliary circuit includingan impedance connected between the said auxiliary electrode of saidiirst device and the positive terminal of said source of direct current,said impedance being of such magnitude that the signal voltage developedthereacross is substantially greater than the signal voltage applied tothe control element of said first device, and also of such magnitude asto insure substantially linear operation of said auxiliary circuitwithin the desired operating range, and means for deriving from saidauxiliary circuit a signal voltage less than the total signal voltagedeveloped across said impedance, but otherwise corresponding thereto,and for impressing said derived signal voltage upon the control elementof the second space dischargedevice.

4. A phase inverter system for a push-pull amplifier, comprising a pairof space discharge devices, each having an anode, a cathode, and acontrol element, and at least the iirst of said devices having anauxiliary electrode interposed between the control and anode elements, aload circuit connecting the anodes oi said devices in push-pullrelation, connections for applying an input signal to the controlelement of said first device, an auxiliary circuit interconnecting theauxiliary electrode of said first device with the cathode of saidrstdevice and including a source of direct current, said auxiliarycircuit including an impedance connected between the said auxiliaryelectrode of said ilrst device and the positive terminal of said sourceof direct cur-l rent, said impedance being of such magnitude that thesignal voltage developed thereacrossis substantially .greater than thesignal voltage applied to the control element of said first device, andalso of such magnitude as to insure substantially linear operation ofsaid auxiliary circuit within the desired operating range, and aconnection from a predetermined intermediate point of said impedance tothe control element ci the second space discharge device for supplyingthereto a portion only of the signal voltage across said impedance. saidintermediate point being so chosen that the said portion issubstantially equal to the signal voltage applied to the control elementoi said first device.

5. A phase inverter system for a push-pull ampliiler, comprising a pairof space discharge devices, each having an anode, a cathode, and acontrol grid'. and at least the ilrst of said devices having a screenelement interposed between the control grid and anode, a load circuitconnecting the anodes of said devices in push-pull relation, connectionsfor applying an input signal to the control grid of said nrst device, anauxiliary circuit interconnecting the screen element of said rst devicewith the cathode of said rst device and including a source of directcurrent, said auxiliary circuit including an impedance connected betweenthe screen element of said rst device and the positive terminal oi saidsource of direct current, said impedance being of 'such magnitude thatthe signal voltage developed thereacross is substantially greater thanthe signal voltage applied to the control grid of said first device, anauxiliary circuit interconnecting the auxiliary electrode of said firstdevice with the cathode of said rst device and including a source ofdirect current, said auxiliary circuit including a connection from apredetermined intermediate point of said impedance to the control gridof the second space discharge device for supplying thereto a portiononly of the signal voltage across said impedance, said intermediatepoint being so chosen that the said portion is substantially equal tothe signal voltage applied to the control grid of said first device.

6. A phase inverter system for a push-pull amplifier, comprising a pairof space discharge devices, each having an anode, a cathode, and acontrol element, and at least the rst of said devices having anauxiliary element interposed between the control and anode elements, aload circuit connecting the anodes of said devices in push-pullrelation, connections for applying an input signal to the controlelement of said irst device, an impedance connected between the saidauxiliary element of said flrst device and the pcsitive terminal of asource current, said impedance being of such magnitude that the signalvoltage developed thereacross is substantially greater than the signalvoltage applied to the control element lof said first device, and avoltage divider for applying to the control element of the second spacedischarge device a portion only of the signal voltage across saidimpedance, said portion being substantially equal to the signal voltageapplied to the control element of said rst device.

' WALTER E. GILBERT.

